 \begin{tikzpicture}[
node distance = 2cm and 2cm,
     N/.style = {circle, draw, font=\Large, 
                 minimum size=1.1em},
       every edge/.style = {draw, -Stealth, semithick, shorten >=2pt},
every edge quotes/.style = {auto=right, inner sep=2pt}
                        ]
\node[N] (a)[label=left :{+ -}] {1};
\node[N] (b)[right of=a] {2};
\node[N] (c)[right of=b][label=right :{-}]{3};
\node[N] (d)[below of=b][label=right :{-}]{4};
\node (a1) at ([xshift=-2em,yshift=2em]a.north){如右图有限自动机:};
\node(c1)[draw,rectangle callout,callout relative pointer={(1,-1)},rounded corners,align=center] at ([xshift=-8em,yshift=0em]a.west) {则L(FA)的\\ \color{red}识别\color{black}过程如下所示:};
\path
(a) edge [above] node {a} (b)
    edge [above] node {b} (d)
(b) edge [above] node {c} (c)
    edge [loop above] node {b} ()
(d) edge [loop above] node {b} ()
    ;
    \end{tikzpicture}